1. Field of the Invention
The present invention relates to an apparatus and a device manufacturing method.
2. Description of the Related Art
To manufacture devices (e.g., a semiconductor integrated circuit device and a liquid crystal display device), an exposure apparatus of a static exposure type, such as a stepper, and an exposure apparatus of a scanning exposure type, such as a step-and-scan exposure apparatus, are used.
FIG. 13 shows the configuration of an exposure apparatus of the related art. An illumination optical system IL illuminates a reticle RT serving as an original with light from an exposure light source LS. The reticle RT is provided with a fine circuit pattern to be transferred. An image of the pattern is projected onto a wafer W by a projection optical system PO.
The projection optical system PO is to have as large a resolving power as possible. To increase the resolving power, the numerical aperture of the projection optical system PO is set to be high, and as a result, the depth of focus of the projection optical system PO is considerably small. For this reason, the height of a surface of the wafer W is measured by a focus detection system FS using an oblique incidence detection method, and the wafer W is moved so that the surface of the wafer W coincides with the focal point of the projection optical system PO, as disclosed in Japanese Patent Laid-Open No. 2000-21768.
With improvement in resolving power, there is a demand to increase the alignment accuracy. For that purpose, an alignment mark on the wafer W is observed at a plurality of positions with an alignment detection system OA provided outside the optical axis of the projection optical system PO, whereby the position of the alignment mark in a plane perpendicular to the optical axis of the projection optical system PO is measured. In this case, there is a distance between the optical axis of the projection optical system PO and the optical axis of the alignment detection system OA. The distance is referred to as a base line amount. If the base line amount changes, when the wafer W is moved below the projection optical system PO after the alignment mark is measured with the alignment detection system OA, an error is caused. Accordingly, the change in the base line amount is measured and corrected by a TTL calibration system (not shown). This realizes more accurate and stable alignment.
The focus detection system FS projects an image of an index mark provided therein onto the wafer W, refocuses the image reflected by the wafer W onto a light receiving sensor, and measures the height of the surface of the wafer W on the basis of the change in the imaging position on the light receiving sensor. The focus detection system FS itself cannot check the position of the image of the index mark on the wafer W.
The focus detection system FS uses a light source that generates light with a plurality of wavelengths (broadband wavelengths). In the focus detection system FS, therefore, the imaging position of the index mark may differ according to the wavelength because of the differences in refractive index of the lens of the optical system for the wavelengths (chromatic aberration).
Further, the imaging position of the index mark may deviate from the designed position because of aberration remaining in the optical system of the focus detection system FS and an installation error of the focus detection system FS.
In the exposure apparatus, the positions of points (positions in the xy-direction) on the wafer in a plane perpendicular to the optical axis of the projection optical system PO are measured by the alignment detection system OA, and the surface height (position in the z-direction) of each point on the wafer are measured by the focus detection system FS.
If the position of the image of the index mark deviates from the designed position, the position in the xy-direction measured by the alignment detection system OA does not coincide with the position in the xy-direction on the wafer W whose surface height is measured by the focus detection system FS.
For this reason, even when the wafer W is moved directly below the projection optical system PO on the basis of the position in the xy-direction measured by the alignment detection system OA and the surface of the wafer W is driven to the focal position of the projection optical system PO on the basis of the surface height measured by the focus detection system FS, in actuality, the wafer W is not exposed at the focal position, but is exposed at a defocused position. This may worsen exposure performance.